This has pretty much been the case with every region yet, yeah. This is obviously so that Ash won't have one overpowered Pokémon with him throughout the regions that would be there to save his butt everytime he was in a tough situation, though of course he still needs to keep his partner. I mean just look at Charizard. He has been one of Ash's Pokémon for almost as long as Pikachu, though he's still as strong as ever because it doesn't matter that much, since he's not a part of Ash's main team in the newer regions, except for in the league battles.

Though on the actual topic, the fifth generation was supposed to be a restart of everything, so it makes sense for Ash to be weaker than his rival at first, then slowly climbing further up to becoming stronger than him, and obviously Pikachu should've been much stronger than all of Trip's Pokémon, though that would've made it less of an experience for the viewers, and less of a rival if Ash was always able to beat him. Of course they could turn the roles around, so that Ash kept on beating his rival and then the rival got the better of him in the league, though since Pokémon is aimed for the younger audience, and since that's probably not what they want to see, I don't think the possibilities of that ever happening are that big. But yeah, Ash will grow stronger, and his team will most likely make it further in the Unova league than they've done in any other league, since Ash appears to get further and further in every new league he competes in.

All in all, just stop worrying about Ash becoming weaker. Of course it doesn't make sense, but how else are they gonna show Ash progressing if he's already as good as they get?